Display screen or portion thereof with a transitional graphical user interface for a gaming system



FIG. 1 is a front view of a first image in a sequence of a first embodiment of a display screen or portion thereof with a transitional graphical user interface for gaming system showing my new design;

FIG. 2 is a front view of a second image thereof;

FIG. 3 is a front view of a third image thereof;

FIG. 4 is a front view of a first image in a sequence of a second embodiment of a display screen or portion thereof with a transitional graphical user interface for gaming system showing my new design;

FIG. 5 is a front view of a second image thereof;

FIG. 6 is a front view of a third image thereof;

FIG. 7 is a front view of a first image in a sequence of a third embodiment of a display screen or portion thereof with a transitional graphical user interface for gaming system showing my new design;

FIG. 8 is a front view of a second image thereof; and,

FIG. 9 is a front view of a third image thereof.

The outer broken line rectangle shows the display screen or portion thereof and forms no part of the claimed design. The broken lines illustrating a finger show environment only, form no part of the claimed design. All other broken lines show portions of the graphical user interface and form no part of the claimed design. 

CLAIM The ornamental design for a display screen or portion thereof with a transitional graphical user interface for a gaming system, as shown and described. 